As you can see you can push the system upto 16GB of RAM using two 8GB memory modules.

One of the issues I often see when someone is asking about adding more RAM is a hard drive is in trouble! So before you upgrade your RAM how about making sure your system is in good shape.

Here are the things I would do in order:

Make a good backup!

Using a cache & log cleaning app clean out the old cache & log files

Clean out any old stuff one does not need. Basically you want to leave a minimum of 1/4 of the drive free and if you can 1/3 (1/3 for smaller drives).

From an external bootable drive run Disk Utility to fix any permission & disk errors.

I would also recommend running a drive de-fragment app like Drive Genius

Then and only then would I consider upping the RAM.

In most cases I would stay at 8 GB that should be enough. If you are working on very large sets of data (DB, image or video processing) or running a lot of apps concurrently then going to 16GB would make sense, otherwise it doesn't offer any benefit.